The application of coatings onto various substrates by the use of spray guns is well known in the prior art. This spraying typically has been accomplished in several different ways, including the following: (1) conventional air atomized; (2) airless spray (high pressure fluid through an orifice); (3) air assisted (a combination of (1) and (2)); and (4) low pressure, high volume (LPHV) air. LPHV air (also known in the industry as high volume, low pressure (HVLP) air) is normally less than 15 psig at a temperature well in excess of the ambient temperature. The term LPHV air as used in this application shall mean any warm air supply to a spray gun which has lower pressure than that found in conventional air atomized spraying or air assisted spraying. The LPHV route to atomize the coating is gaining increasing acceptance over the other methods because of the following potential advantages: (1) higher transfer efficiency because of the low atomizing pressure minimizing over-spray and and bounceback; (2) improved spray quality because the spray pattern can be precisely controlled; (3) the soft delivery prevents paint from being forced under masks; (4) the warm air is especially beneficial for atomizing high-solids paint; (5) the low pressure arrangement produces small particle sizes and is less prone to disturb the relationship of solvent to pigment/binder; (6) the laminar style flow provides a confined pattern that can effectively penetrate into hard to reach areas; (7) spray areas are cleaner because of better spray efficiency; and (8) the ability to drastically lower the air volume and pressure in order to do excellent texture finishes.
In the past, there have been two basic methods of supplying LPHV air to a spray gun in order to atomize the coating. The most common method is the turbine. In fact LPHV spraying is alternately called "turbine spraying". Turbine spraying uses a high performance turbine/compressor which intakes filtered ambient air and creates warm LPHV air. (The heating of the air stream is a natural byproduct of high performance turbines). The second LPHV method involves a compressed air "conversion unit". This is simply a common pressure regulator that reduces the compressed air pressure down to 5-15 psig. This low pressure air is then heated to approximately 100.degree.-200.degree. F. by means of an electric resistance heater.
Both of the above-mentioned LPHV methods have certain drawbacks. For instance the turbine method has moving parts that can break down and that require occasional maintenance. It also requires electrical power to operate which must be explosion proof for electrically hazardous classified areas. Also the control of air temperature is not precise with turbines. In most cases the temperature is only controlled by the length of air hose connected between the turbine and the spray gun. And, the initial capital costs are relatively high for a turbine system. Likewise the compressed air conversion unit suffers from similar drawbacks. The temperature control on these units is effected by the use of a thermostat utilizing on-off control and resulting in significant temperature cycling. In order to make this type of unit suitable for electrically hazardous areas expensive purging or an expensive explosion proof mounting box would be required.
What is needed is a source of LPHV air for paint spray guns which is inexpensive, requires little mainainence, has simple and precise temperature adjustment and is explosion proof.